A 1300 nm zero dispersion fiber network is known to be used providing highly reliable optical communications. At a receiving side of a network zero dispersion of optical signals having a wavelength of 1300 nm is obtainable.
As of late, wavelength division multiplex communications for transmitting optical transmission signals (optical pulse transmission signals) having a plurality of wavelengths is being implemented by using this existing 1300 nm zero dispersion fiber network. In wavelength division multiplex communication using a wavelength of approximately 1550 mm through the existing 1300 mn zero dispersion transmission network, wavelength dispersion of approximately 17 ps/km-nm occurs causing a disturbance in long range transmission. Generally, wavelength dispersion includes positive dispersion and negative dispersion; negative dispersion refers to a phenomenon that, as the wavelength becomes larger, the group index of the optical transmission fiber lessens and the group velocity of the transmission signal increases and the pulse width increases in response to this increase of the group velocity; positive dispersion refers to a phenomenon that, as the wavelength increases, the group index of the optical transmission fiber increases and the group velocity of the transmission signal lessens and the pulse width increases in response to this decrease of group velocity.
An ordinary existing 1300 nm zero dispersion transmission network has a dispersion of approximately 17 ps/km-nm at a wavelength of approximately 1500 nm as described above. In a long range transmission of, for example, 100 km distance, there is a problem that dispersion of approximately 1700 ps/km-nm occurs at the receiving side of an optical transmission and, even though high density/high speed communication is attempted by minutely dividing the wavelength at approximately 1550 nm, a signal of one side wavelength is superposed with a signal of the other side wavelength and separation of signals is difficult since dispersion is large as described; therefore due to this overlapping of adjacent channels, optical communication performance is worsened.
In the prior art, a dispersion compensating optical fiber for compensating a quantity of chromatic dispersion of a specific wavelength is inserted into the optical transmission path to prevent the increase in the quantity of chromatic dispersion as is described above.
This type of dispersion compensating optical fiber has negative dispersion and the increase of the quantity of dispersion of a specific wavelength in an optical transmission is lessened by offsetting positive dispersion of a 1300 nm zero dispersion transmission network with negative dispersion by utilizing this dispersion compensating optical fiber.
Dispersion compensating optical fiber includes five types of refractive index profiles as disclosed in Japanese Patent Application Disclosure HEI 6-11620. These five types of refractive index profile are shown in FIG. 5. In the refractive index distributions shown in FIGS. 5a and 5b, the dispersion slopes (the derivative of dispersion with respect to wavelength) respectively have a positive value and, in the use of such dispersion compensating fiber, dispersion compensation can be carried out for a specific wavelength; however this dispersion compensating optical fiber is unsuitable for other wavelengths as a compensation optical fiber for wavelength division multiplex transmission since the quantity of dispersion increases with wavelength. The three types of optical fibers relating to FIGS. 5c to 5e may have a refractive index having a negative dispersion slope. Though the W-shaped refractive index profile shown in FIG. 5c has long been examined, conventional W-shaped optical fiber has been able to provide a negative dispersion slope but has required an extremely long fiber length necessary for a dispersion compensation since the quantity of negative dispersion of the conventional W-shaped optical fiber has been small, and therefore it has been unsuitable for practical use. It is better to reduce the diameter of the core to increase the quantity of negative dispersion; if the core diameter of the optical fiber is reduced and the quantity of negative dispersion is increased, the dispersion slope in the W- shaped refractive index profile is inverted from a negative slope to a positive slope and therefore the W-shaped optical fiber is unsuitable to wavelength multiplex division transmission.
It is an object of the invention to overcome the above-described problem of the prior art by providing a structure capable of simultaneously having a negative dispersion slope with an effective size and a negative dispersion with an appropriate magnitude in the W-shaped refractive index distribution to provide a dispersion compensating optical fiber for wavelength division multiplex transmission which enables compensation of dispersion in a wide range of wavelengths at or about approximately 1550 nm, and wavelength division multiplex transmission by using the existing 1300 nm zero dispersion transmission network.
A dispersion compensating optical fiber for wavelength division multiplex communication wherein wavelength dispersion .sigma. in range of a0.ltoreq.a.ltoreq.a1 is controlled in a range of .sigma..ltoreq.-100 ps/km-nm when it in assumed that a core radius with which a wavelength dispersion slope (d.sigma./d.lambda.) is zero is a0 and a core radius with which the wavelength dispersion slope (d.sigma./d.lambda.) is -0.28 ps/km-nm.sup.2 is al in a that a core radius of an optical fiber is a, wavelength dispersion is .sigma., and a wavelength of optical transmission signal is .lambda..
The present invention is adapted as described below to attain the above object. Specifically, the present invention is characterized in that the wavelength dispersion .sigma. in range of a0.ltoreq.a.ltoreq.a1 is set to be within in a range of .sigma..ltoreq.-100 ps/km-nm when it in assumed that a core radius in a case that the wavelength dispersion slope (d.sigma./d.lambda.) is zero is a0 and a core radius in the case that the wavelength dispersion slope (d.sigma./d.lambda.) is 0.28 ps/km-nm.sup.2 is a 1 if the core radius of an optical fiber is a, wavelength dispersion is .sigma., and a wavelength of optical transmission signal is .lambda..
The present invention is also characterized in that the refractive index structure of the above optical fiber has the W-shaped refractive index profile, an internal clad layer is formed outside the core, an outermost clad layer is formed on the outside of the internal clad layer, a dopant for reducing the refractive index is doped in the internal clad layer so that a specific refractive index difference is -0.45%, the outermost clad layer being made of pure silica and the dopant for raising the refractive index is doped in the core so that the specific refractive index difference is +2.8%, the diameter ratio of the core to the internal clad layer is determined to be within the range of 1:1.5 to 1:4.0, the dispersion having a negative slope at a wavelength of about 1550 nm, and the wavelength dispersion at optical wavelengths of about 1550 nm is smaller than -100 ps/km-nm and larger than -170 ps/km-nm in a small range wherein the core diameter is larger than 2.1 .mu.m and smaller than 2.3 .mu.m.
In the above configuration according to the present invention, if the dispersion compensating optical fiber for wavelength division multiplex transmission according to the present invention is inserted into, for example, an existing 1300 nm zero dispersion transmission network and wavelength division multiplex communication is carded out with at a wavelength of approximately 1550 nm, optical signals of respective wavelengths which have reached the terminal through the 1300 nm zero dispersion transmission network have large quantities of wavelength dispersion. However, since the dispersion compensating optical fiber according to the present invention simultaneously has the high negative chromatic dispersion and a negative dispersion slope, this offsets a large unwanted positive dispersion quantity, which occurs through the 1300 nm zero dispersion transmission network; Effectively optical signals of respective wavelengths which have passed through the dispersion compensating optical fiber according to the present invention have dispersion values almost equal to zero. Consequently, separation of signals with respective wavelengths is certainly carried out at the receiving side to enable high density/high speed wavelength division multiplex communication in high reliability.
The present invention provides a new dispersion compensating optical fiber which has a negative dispersion slope and a negative high dispersion, that effectively offsets large dispersion quantities caused in the optical transmission path and receives signals with small-wavelength dispersion at the receiving side by utilizing optical fiber having high negative dispersion according to the present invention.
Thus the reliability of high density/high speed wavelength division multiplex communication can be substantially raised.
The dispersion compensating optical fiber according to the present invention has negative high dispersion and, even when large positive dispersion occurs in optical transmission signals which have passed through the optical transmission path, the positive dispersion can be compensated with a short length of optical fiber. Accordingly, the dispersion compensating optical fiber can be housed in a small compact package and therefore excellent in practical use.
In addition, in the wavelength division multiplex communication using the wavelength of approximately 1550 nm with an existing 1300 nm zero dispersion transmission network, wavelength dispersion of transmission optical signals of various wavelengths can be effectively offset and compensated at the receiving side by inserting the dispersion compensating optical fiber according to the present invention into the optical transmission path, thereby achieving high density/high speed wavelength division multiplex communication with high reliability at wavelength of or about approximately 1550 nm.
The optical fiber is subject to a condition for effective propagation of light. This light propagating condition depends on the effective refractive index (.beta./k), where .beta. is a propagation constant within the waveguide and k is the number of waves in the media space.
In the optical fiber having the W-shaped refractive index profile, the effective refractive index of light signals depend on the values of specific refractive index difference .DELTA.+ of the core and specific refractive index difference .DELTA.- of the internal clad and it is necessary to find an optimum combination of these specific refractive index differences .DELTA.+and .alpha.-.
According to the studies of the present inventor, the propagation conditions tend to be satisfied with a larger specific refractive index difference .DELTA.+ of the core and a smaller specific refractive index difference .DELTA.- of the internal clad and particularly the optimum propagation conditions are obtained --from a-- combination of the specific refractive index difference .DELTA.+ of +2.8% of the core and the specific refractive index difference .DELTA.- of 0.45% of the internal clad. The light propagation performance of the optical fiber deteriorates as the above refractive index differences deviate from the optimum propagation conditions. for example, in case of the optical fiber with .DELTA.+=+2.8% and .DELTA.-=-0.7%, .DELTA.- is too large to deteriorate and in case of the optical fiber with .DELTA.+=2.1% and .DELTA.-=-0.35%, the -light propagation performance similarly deteriorates since .DELTA.+ is excessively small.
As in the present invention, the optimum refractive index for propagation of light can be obtained by applying .DELTA.+=+2-8% and .DELTA.-=-0.45%.